What do you mean by overflow valve?
Fuel return system
The fuel return system starts at the overflow valve that is on the engine side of
the injection pump even with the first delivery valve. The overflow valve looks
like a banjo fitting, but it contains a pressure valve. The type shown in the
picture has a spring, a steel ball, and a washer shaped like a tiny derby hat to
regulate pressure in the injection pump. This is essential so there will be adequate
fuel flow into the injection pump plungers when the fill ports open. The line from this
valve goes to the rubber return line discussed in “Fuel supply and return lines”
above. If there is low fuel pressure that is caused by a weak or defective overflow valve it is easy
to check. While the engine is running have someone watch the fuel pressure gauge. Clamp the
rubber return line closed with some pliers. If the low pressure is caused by this valve the pressure
read by the pressure gauge will increase very quickly. Do not hold the return line closed very
long to prevent damage to the fuel system. If the overflow valve is the cause and is the type
shown in the picture it may be temporarily repaired. Remove it from the injection pump. When
you do this cram a shop rag below the valve to catch any washers you may drop. If they get down
there you may never see them again. Take the entire valve off. Don’t remove the small screw
until you have the valve out and on your work bench. The spring in the overflow valve is
supposed to be a little bit longer then ½ inch, 0.550". If it is shorter you can stretch it a little. Just
a little bit goes a long ways. If you stretch it more than a little it will wreck the spring so it will be
worthless. The steel line from the overflow valve may crack and leak.
